Method for treating cyanide waste liquid

ABSTRACT

A highly reliable detoxifying method of the cyano matters is provided. The waste-liquor ( 1 ), which contains at least one of free cyano matters, a cyano complex and a reducing compound exhibiting volatility in alkaline aqueous solution, is heated under an alkaline condition to a temperature range, which lies within a range of from room temperature to boiling point, and which includes a high temperature range of 80° C. or more, followed by holding the temperature ( 7 ); oxidation-reduction potential of the waste liquor is measured from the room temperature; and, hypochlorite ( 2 ) is intermittently or continuously added to the waste liquor from the room temperature, until the oxidation-reduction of the the hypochlorite is detected.

TECHNICAL FIELD

The present invention relates to a detoxification treatment method ofprocess-water, waste-water, waste liquor and the like, containing acyano complex and the like.

BACKGROUND TECHNIQUE

The alkali chlorine method, which is broadly carried out in thedetoxification treatment of waste-water containing cyanides, has twosteps for decomposing cyanides, i.e., an addition of chlorine under analkaline condition; and, subsequently, a conversion of pH to neutral anda further addition of chlorine. A sodium-hypochlorite aqueous solutionis usually used as the chlorine, and the dosage control by means of anORP meter is carried out as follows.

First Step Reaction: pH 10˜12. The oxidation-reduction potential of300˜350 mVNaCN+NaOCl→NaCNO+NaCN  (1)

Second Step Reaction: pH 7˜8. The oxidation-reduction potential of600˜650 mV2NaCNO+3NaOCl+H₂O→N₂+3NaCl+2 NaHCO₃  (2)

According to “Techniques and Regulations of Pollution Prevention, WaterChapter, 5^(th) Edition” supervised by the Environment and LocationBureau of the Ministry of Trade and Industry, pages 261˜262), the freecyano matter and cyano complexes of zinc and copper can be oxidized anddecomposed by the alkaline chlorine method under dosage control using anORP meter. Contrary to this, the cyano complexes of nickel and silvercan be decomposed solely by prolonged reactions with excessive chlorine.Consequently, the chlorine must be dosed in a constant amount exceedingthat required for decomposition. The dosing control by an ORP meter is,thus, impossible. Furthermore, since complexes of iron, cobalt and gold,which form stable complexes, are also stable under the presence ofexcessive chlorine, their decomposition is allegedly difficult by thealkaline chorine method.

The cyano matters are designated as harmful material in the PreventionLaw of Water Pollution, and are defined as the total cyano mattersincluding the cyano complex. Treatment methods of the cyano waste-liquorincluding these stable cyano complexes have been developed.

According to the method of Japanese Unexamined Patent Publication No.49-1058, the cyano waste-liquor, which contains a cyano complex, isheated in a pressure vessel to 150° C. or higher so as to hydrolyze thecyano complex, and the dissociated free cyano matters are thermallydecomposed into ammonia and formic acid. This method is referred to asthe thermal hydrolyzing method. The formic acid, which is adecomposition product of this method, is a COD component of the wasteliquor, and the ammonia is subjected to the nitrogen regulation. Asecondary treatment is, therefore, necessary.

A method developed for obviating the need for the secondary treatment,is the wet oxidizing method (Japanese Unexamined Patent Publication No.7-116672). In this method, an oxidizing process is additionally carriedout during the reaction in the pressure vessel as described hereinabove.According to this method, the cyanides are oxidized and decomposed intocarbon dioxide and nitrogen.

The two types of methods described hereinabove are based on physicalchemical reactions and are therefore highly reliable in detoxifyingcyanides. However, since each of these two types is a high-pressuretreatment, the investment cost is enormous.

When the waste-water containing cyano complexes is treated by thethermal hydrolysis method or the wet oxidizing method, the metal, whichis converted to a complex by the cyano matters, is precipitated anddeposited in the form of hydroxide and oxide, resulting in the formationof sludge. In order to detoxify the waste-water, which is incorporatedin the sludge, mechanical stirring becomes necessary. Installation of astirring means in a high-pressure vessel incurs considerable increase ofinvestment cost because the sealing characteristics of the vessel mustbe ensured. This is the reason that hinders the employment of thosemethods.

The present inventors carried out a document search in a broad range tofind such treatment methods of waste-water containing cyano complexesthat require minimum investment costs and are highly reliable. As aresult, the present inventors focused on Japanese Examined PatentPublication No. 50-118962. This publication describes that waste-liquorcontaining an iron cyano-complex is heated and detoxified by means ofdosing the oxidizing agent in a constant amount.

According to the method of Japanese Unexamined Patent Publication No.50-118962, the pH value of waste-liquor containing an iron cyano-complexis adjusted to approximately 10˜11, an oxidizing agent (hypochlorite) isadded, and the reaction is carried out while maintaining the liquortemperature at approximately 80˜95° C. Allegedly, the iron cyano complexis converted to iron oxide under the pH and temperature conditionmentioned above and precipitates. Further, the cyano matters aredecomposed into carbon dioxide and nitrogen.

However, notwithstanding a lapse of 25 years from the invention ofJapanese Unexamined Patent Publication No. 50-118962, use of this methodvirtually has not spread. It can be said that persons skilled in thisfield do not recognize at all this method to be capable of treatingcyano waste-liquor, which contains stable cyano complexes of not onlythe iron mentioned above but also nickel, silver, cobalt and goldcomplexes.

The present inventors made a tracing test and thus confirmed that thewaste-liquor containing an iron cyano complex was detoxified asdescribed in the specification.

The present inventors presume that the above method has not been putinto practice for the following reasons.

{circle over (1)} Japanese Unexamined Patent Publication No. 50-118962is the so-called chlorine treating method, in which the oxidizingdecomposing always proceeds under an excessive amount of an oxidizingagent. The end point of the reaction is, therefore, not distinct. Thismethod is, therefore, not reliable in the detoxifying the cyanides.

{circle over (2)} The hypochlorite present in excess self-decomposes ata temperature of 80° C. or more. The amount of the reagent used becomestherefore excessive.

DISCLOSURE OF INVENTION

The present inventors directed their attention to the techniques of theheating-type alkali chlorine method (Japanese Unexamined PatentPublication No. 50-118962), which pertains to the detoxification ofcyano waste liquor, such as the rinsing waste-liquor of the salt-bathnitriding treatment and plating waste liquor, and which involves lowinvestment cost. Particularly, in a method for oxidizing and decomposingthe waste-liquor, which contains such stable cyano complexes as iron,cobalt and gold complexes, is maintained in a temperature range of 80°C. or more, and is oxidized and decomposed by the chlorine, the presentinventors have established a method, which does not rely on the constantdosage resulting in excessive chlorine but which enables dosage controlby the ORP meter.

As a result, the present inventors could solve the problems mentionedabove by the discovery of an invention. This invention is related to anoxidizing decomposition method of waste liquor containing a cyanocomplex by means of adding hypochlorite under alkaline condition and ischaracterized in that said waste liquor, which contains at least onecyano complex of iron, cobalt and gold, is maintained within atemperature range of from 80° C. to boiling point, and further thehypochlorite is continuously or intermittently added until the wasteliquor arrives at the oxidation-reduction potential of the hypochlorite.

In addition, it is possible to continuously or intermittently add thehypochlorite in to the waste liquor, which further contains at least oneof ammonia and free cyano matter, within a temperature range lower than80° C., until the waste liquor arrives at 400 mV of theoxidation-reduction potential, and, subsequently, the temperature iselevated up to a range of from 80° C. to boiling point, and thehypochlorite is added until the oxidation-reduction potential of thewaste liquor arrives at the oxidation-reduction potential ofhypochlorite.

When the waste liquor is at the room temperature, it is preferable thatthe temperature of waste liquor is elevated from the room temperature toa temperature within 80° C. to boiling point followed by maintaining thetemperature, and, further, the oxidation reduction potential of thewaste liquor is measured from the room temperature.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is the oxidation-reduction potential curves of a solution ofNaCN, NaCNO, Na₄Fe(CN)₆, (NH₄)₂SO₄, alone.

FIG. 2 is the oxidation-reduction potential curves of a mixed solutionof NaCN, NaCNO, Na₄Fe(CN)₆, and (NH₄)₂SO₄.

FIG. 3 is a graph showing the time change in the oxidation-reductionpotential and temperature.

FIG. 4 is a graph showing the time change in the CN, NH₃ and Feconcentrations and pH.

FIG. 5 shows the apparatus and flow for carrying out the method of thepresent invention.

FIG. 6 is a graph showing the oxidation-reduction potential in anexample, in which the copper-plating waste-liquor is detoxified.

The present invention is hereinafter explained mainly with regard to anexample of the salt-bath nitriding waste liquor in the followingsequence. Most of the processing water, waste liquid, waste liquor andthe like, that is, the processing subject of the present invention, areas follows: {circle over (1)} waste liquor of salt-bath nitriding method(components—total cyano matters, free cyano matters, reducing-typecompound exhibiting volatility in the alkaline aqueous solution);{circle over (2)} plating waste liquor (total cyano matters, free cyanomatters). Hereinafter, an example of the intoxication of the wasteliquor of salt-bath nitriding method is mainly described. Treatment of{circle over (3)} general waste water component—reducing type compoundexhibiting volatility in the alkaline aqueous solution) is described asreference.

-   -   (1) Reaction Formulae    -   (2) Oxidation-Reduction Potential    -   (3) Temperature Dependence of Reactions    -   (4) pH Range    -   (5) Detection of Oxidation-Reduction Potential and Temperature    -   (6) Two-Step Method    -   (7) Single Step Method    -   (8) Theoretical Amount of Sodium Hypochlorite    -   (9) Regulated Value of Waste Water    -   (10) Detoxification of Solid Discard    -   (11) Decomposition of Valuable Metal Cyano-Flux    -   (12) Best Mode For Carrying Out Invention        -   (a) Example 1 (two-step method, rinsing waste liquor of salt            bath nitriding line)        -   (b) Example 2 (two-step method, rinsing waste-liquor of salt            bath nitriding line)        -   (c) Example 3 (two-step method, removed sludge)        -   (d) Reference Example 1 (two-step method, plating            waste-liquor)        -   (e) Comparative Example 1 (conventional method, excessive            chlorine method)        -   (f) Comparative Example 2 (ditto)        -   (g) Comparative Example 3 (ditto)        -   (h) Example 4 (single step method, rinsing waste liquor of            salt bath nitriding line)        -   (i) Example 5 (single step method, removed sludge)        -   (j) Reference Example 2, Examples 6,7 (single step method,            plating waste-liquor, mixed waste liquor of plating waste            liquor and Tufftride waste liquor, and low-concentration            waste-liquor)        -   (k) Comparative Examples 4, 5, 6 (addition, methods of            sodium hypochlorite: addition of the theoretical amount as a            whole directly after heating, addition of the theoretical            amount as whole before heating, and charging twice as high            as the theoretical amount)        -   (l) Reference Example 3 (Treatment of Copper-Plating            Waste-Liquor)        -   (m) Example 8 (Aging)    -   (13) Applicability in Industry

Reaction Formulae

Salt-bath nitriding is a surface hardening method, in which iron partsare immersed in a molten salt-bath mainly composed of alkali cyanate andalkali carbonate and heated to approximately 580° C. to form a nitridedlayer on the surface of the iron parts. When the iron parts are immersedin the salt bath mentioned above, the cyanic acid in the salt bathdecomposes due to the catalytic reaction of the steel surface followingthe reaction below. The resultant nitrogen in the nascent state diffusesinto the solid steel thereby advancing the nitriding.5NaCNO→3NaCN+Na₂CO₃+CO₂+2N  (3)

Along with the treatment the cyanide formed by the reaction (3)accumulates in the salt bath.

In order to remove the salt deposited on the surface of the steel partstreated in the salt bath, they are rinsed in the subsequentwater-rinsing process. The water-rinsing waste-liquor from thewater-rinsing process contains, therefore, the sodium cyanate and alkalicarbonate, which are the original salt-bath components, andadditionally, cyanide (free cyano matters), ferrocyanides (cyanocomplexes) formed due to reaction between the cyanide and the steelsurface, and ammonia, which is a decomposition product of the cyanateunder the hydrolysis reaction (4)NaCNO+2H₂O→CO₂+NH₃+NaOH  (4)

Such chlorine compounds as chlorine gas, bleaching powder, andsodium-hypochlorite aqueous-solution can be used, when the cyanocomplexes in the aqueous solution are oxidized and decomposed bychlorine. The chlorine gas is toxic gas and incurs danger duringhandling. The bleaching powder is powder and has difficulty in handlingand is liable to form the calcium-carbonate scale. Its application tothe treatment of waste-water is, therefore, limited. Desirably, thesodium-hypochlorite aqueous-solution is used in the light of safety andoperability.

Therefore, it is presumed that the following reactions may occur, whenthe hypochlorite is caused to react with the rinsing waste-liquordischarged from the salt-bath nitriding.

{circle over (1)} Oxidation of Cyanide to CyanateNaCN+NaClO→NaCNO+NaCl  (5)

{circle over (2)} Decomposition of Cyanate Formed by Equation (5)2NaCNO+3NaClO+H₂O→2CO₂+N₂+2NaOH+3NaCl  (6)

{circle over (2)}′ Decomposition of Unreacted Cyanate of Equation (4)2NaCNO+3NaClO+H₂O→2CO₂+N₂+2NaOH+3NaCl  (6′)

{circle over (3)} Oxidation of Ferrocyanate to Ferricyanate2Na₂Fe(CN)₆+NaClO+H₂O→2Na₃Fe(CN)₆+2NaOH+NaCl  (7)

{circle over (4)} Oxidation of Ferrycyanate to CyanateNa₃Fe(CN)₆+6NaClO+3NaOH→Fe(OH)₃+6NaCNO+6 NaCl  (8)

{circle over (5)} Decomposition of Cyanate Formed by Reaction (8)2NaCNO+3NaClO+H₂O→2CO₂+N₂+2NaOH+3NaCl  (6″)

{circle over (6)} Decomposition of Ammonia2NH₃+3NaClO→N₂+3NaCl+3H₂O  (9)

Oxidation-Reduction Potential

In order to obtain basic data of the dosage control by an ORP meter, thepresent inventors measured an oxidation-reduction potential curve of theabove equations (5) through (9), the inventors prepared solutionscontaining NaCN, NaCNO, Na₄Fe(CN)₆, or (NH₄)₂SO₄, alone. The pH of thesolutions was 11.5. The liquor temperature was maintained at 80° C. Thesolutions were titrated by the sodium hypochlorite solutions and therespective oxidation-reduction curves were obtained as shown in FIG. 1.The inflection points are shown in Table 1 (measurement electrode—aplatinum electrode; the reference electrode is Ag/AgCl electrode). TABLE1 First Inflection Second Inflection Third Inflection Reagents Pointpoint point NaCN  0˜400 mV 550˜700 mV — NaCNO 600˜740 mV — —Na₄[Fe(CN)₆]  0˜220 mV 250˜450 mV 550˜600 mV (NH₄)₂SO₄ 300˜600 mV — —

The first inflection point of NaCN (0˜400 mV) is the reaction end pointof equation (5), in which cyanide is converted to cyanate. Incidentally,the cyanate formed by this reaction is non-toxic.

The second inflection point of NaCN (550˜700 mV) is the reaction endpoint of equation (6), in which cyanate is oxidized to carbon dioxideand nitrogen.

The first inflection point of NaCNO (600˜740 mV) is the reaction endpoint of equation (6), in which cyanate is converted to carbon dioxideand nitrogen as in the second inflection point of NaCN according toequation (6). Upon completion of the reaction, hypochlorite is presentin small excess, so that its oxidation-reduction potential is detected.

The first inflection point of Na₄[Fe(CN)₆] (0˜220 mV) is the reactionend point of equation (7), in which ferrocyanate is oxidized toferricyanate. Upon completion of the reaction, hypochlorite is presentin small excess, so that its oxidation-reduction potential is detected.

The second inflection point of Na₄[Fe(CN)₆] (250˜450 mV) is the reactionend point of equation (8), in which ferricyanate is decomposed intocyanate and ferric hydroxide.

The third inflection point of Na₄[Fe(CN)₆] (550˜600 mV) is the reactionend point of equation (6), in which cyanate is decomposed into carbondioxide and nitrogen as in the second inflection point of NaCN, i.e.,equation (6), and the first end point of NaCNO.

The first inflection point of (NH₄)₂SO₄ (300˜400 mV) is the reaction endpoint of equation (9), in which ammonia is oxidized and decomposed intonitrogen and water.

Temperature Dependence of Reactions

The actual waste liquor is simulated in such a manner that therespective reagents of NaCN, NaCNO, or Na₄Fe(CN)₆ and (NH₄)₂SO₄ aremixed to prepare a solution having 11.5 of pH and 85° C. of temperature.This solution was titrated by the sodium hypochlorite aqueous solution.The oxidation-reduction potential during the titration was measured. Theresult is shown in FIG. 2. Three inflection points were recognized at{circle over (1)} 0˜200 mV, {circle over (2)} 300˜420 mV, and {circleover (3)} 570˜650 mV. These inflection points correspond to thefollowing reactions, respectively.

-   -   Inflection Point {circle over (1)}: oxidation of ferrocyanate to        ferricyanate and oxidation of cyanide to cyanate    -   Inflection Point {circle over (2)}: decomposition of ammonia    -   Inflection Point {circle over (3)}: decomposition of cyanate    -   It is considered from Tables 1 and 2 that the temperature        dependence of the reactions (5) through (9) is as follows.

Reaction (5) easily proceeds at room temperature.

Reaction (6) is extremely slow in alkali side at room temperature.Therefore, according to the most general detoxifying method of cyanides,i.e., the so-called alkali chlorine method, equations (1) and (5) arecompleted at pH>10, and then the pH is adjusted to 7˜8 so as to performthe oxidation reaction of equations (2) and (6). This method is,therefore, referred to as the alkali-chlorine two-step method. When thepH is kept as pH>10 and temperature is elevated (80° C. or more), thereaction proceeds.

Reaction of equation (7) easily proceeds at room temperature.

Reaction of equation (8) does not take place at all at room temperature,but proceeds at 80° C. or more. Therefore, the decomposition offerricyanide to cyanic acid is carried out at a temperature of 80° C. ormore according to the present invention.

Reaction efficiency of equation (9) is low in an alkaline state at roomtemperature due to dissociation of hypochloric acid. The reactionproceeds at 60° C. or more.

Contrary to the inventors' discovery that reaction (5) easily proceedsat room temperature, Japanese Unexamined Patent Publication No.50-118962, which discloses the heating-type alkali chlorine method,describes that a reaction to form hypochloric acid from hypochlorite(equation (4) in the publication) proceeds slowly at 80° C. or lower(page 272, right and lower column, lines 7˜8 of the publication). Thepresent inventors confirmed that the oxidation of ferrocyanate toferricyanate takes place at low temperature. However, in JapaneseUnexamined Patent Publication No. 50-118962, the oxidizing agent is notadded at low temperature but is added at high temperature of 80˜95° C.(c.f., page 272, right and upper column—left and lower column, line 2)

pH Range

When the waste-water containing the cyano complexes is subjected todecomposition by sodium hypochlorite, pH must be maintained in analkaline side. When the waste water is acidic, there is a danger thatthe sodium hypochlorite is decomposed, thereby generating chlorine gas.

In most cases, the waste-water contains free cyano matters. The aqueoussolution containing the free cyano matters must be maintained alkaline(pH 8˜14, desirably 9˜14) so as to prevent the generation of toxic HCN(cyano gas).

For the reasons described hereinabove, the starting water to be treatedby the present invention is necessarily maintained alkaline.

When the cyano complexes contained in the rinsing water of salt-bathnitriding are subjected to decomposition, the oxidation-reductionpotential of the respective reactions is measured. When thedecomposition of the cyano complexes completes, the cyanic acid has beencompletely decomposed, and the potential shows that (550˜740 mV) of thesodium hypochlorite is present in excess. The dosing of the sodiumhypochlorite is stopped when the oxidation-reduction potential reachesthis value, thereby enabling the dosing control by an ORP meter. Theoxidation-reduction potential curve of this reaction is influenced by pHand shows a distinct inflection point in the pH range of 9˜13.5. At lessthan pH 9, the potential shows hunting and is thus instable. At morethan pH13.5, the change of potential is so slight that the inflectionpoint is not distinct and the dosing control becomes difficult. Thewaste-water to be treated must, therefore, be maintained in a pH rangeof 9˜13.5.

Detection of Oxidation-Reduction Potential and Temperature

In the present invention, the following reactions are caused to fullyproceed based on the considerations above, at lower than 80° C. withregard to the waste liquor, which contains cyanide, ferrocyanide andammonia, the reaction of equation (5), i.e., the oxidation of cyanide tocyanate, which proceeds at low temperature, the oxidation-reductionpotential at the completion of the reaction=approximately 250 mV; thereaction of equation (7), i.e., the oxidation of ferrocyanide toferricyanide, the oxidation-reduction potential at the completion of thereaction=approximately 200 mV; and, the reaction of equation (9), i.e.,the decomposition of ammonia, the oxidation-reduction potential at thecompletion of the reaction=approximately 400 mV. As a result, theammonia and free cyano matters, which involve a danger of vaporizing,are preliminarily decomposed at a low temperature, thereby preventingvaporization of the unreacted ammonia and cyano gas from the alkalinewaste liquor. In the low-temperature step described above, one or moreof the plurality of oxidation-reduction potentials mentioned above maybe set as the setting potential, and the oxidation-reductionpotential(s) is detected and the addition of hypochlorite may becontrolled in such a manner that the hypochlorite is added when themeasured potential is lower than the setting potential.

Subsequently, the temperature is elevated to 80° C. or more, and thetemperature is maintained in a high-temperature range of 80° C. or moreand the boiling point or less, so as to carry out the reaction ofequation (6), i.e., the decomposition of the cyanate, which proceedsslowly or does not proceed unless at high temperature, theoxidation-reduction potential at the completion of thereaction=approximately 650 mV; and the reaction of equation (8), i.e.,the decomposition of ferricyanide to cyanate, the oxidation-reductionpotential at the completion of the reaction=approximately 300 mV.Although in the reaction of equation (9), i.e., the decomposition ofammonia, the oxidation-reduction potential at the completion of thereaction=approximately 800 mV; can be completed at low temperature(40˜80° C.), it is possible to complete the reaction of the residualpart of ammonia at high temperature, which reaction does not complete atlow temperature. After the treatment in the low-temperature process, thehypochlorite is again added, thereby preventing its self-decomposition.

The oxidation-reduction potential of the waste liquor after thesalt-bath nitriding is generally −100˜+100 mV in most cases.

In the present invention, the oxidation-reduction potential of thetreated waste liquor is detected while the temperature is elevated andmaintained in the high-temperature range as described above, and theaddition of hypochlorite is ended when and after the oxidation-reductionpotential of hypochlorite is detected.

The oxidation-reduction potential of the hypochlorite under the alkalinestate, which is measured by the measuring electrode and the referenceelectrode mentioned above, is generally 600˜700 mV but may somewhat varydepending on the kind of the treated waste liquor. Preferably, theoxidation-reduction potential is actually measured in the laboratoryprior to the treatment. However, in practice, the hypochlorite iscontinuously added, the temperature-elevation and maintenance iscontinued, and it can be judged that the oxidation-reduction potentialof the hypochlorite is detected, when the oxidation-reduction potentialreaches a constant value in the range of 600˜700 mV, mentioned above.

In addition, the maintenance in the high-temperature range is carriedout by means of holding the temperature of waste liquor in thetemperature-range of 80° C.˜the boiling point, specifically, holding at,for example, 95° C. for 1 hour.

Two-Step Method

In the present invention, the two-step method may be carried out in sucha manner that the reactions (5), (7) and (9) are essentially completedin the low-temperature step at 80° C. or lower, and subsequently theliquor temperature is elevated to 80° C. or more so as to carry out thereactions (6) and (8). Ammonia is decomposed in the low-temperaturestage by this method.

In the heating-type alkali-chlorine method, the top part of adecomposition reaction-tank is open. However, in the method according tothe present invention, the possibility of vaporization of the unreactedammonia is not completely ruled out, therefore, the reactor must be ofsuch a structure that no gas leaks externally.

In the two-step method according to the present invention, the settingtemperature and the setting potential are first input in thelow-temperature mode, and the rinsing waste liquor in the reaction tankis heated from room temperature to the setting temperature.

First, the oxidation-reduction potential is set at the ammoniadecomposition potential of equation (9), which exhibits the highestinflection point in the reactions (5), (7) and (9) of thelow-temperature range. The ammonia decomposition potential is usuallyapproximately 400 mV but may not necessarily be of this value dependingupon the waste liquor. This potential should therefore be preliminarilyconfirmed. The reaction of equation (5) proceeds at room temperature. Inaddition, the reaction of equation (9) is slow at room temperature butis accelerated with the rise of temperature. The dosing of hypochloriteshould therefore be desirably started from room temperature from theviewpoint that the decomposition treatment time is shortened and furthervaporizing of the harmful gases is prevented.

When the reactions of the low-temperature region proceed to such a levelthat the measured potential stably arrives at the setting potential, atthis point, the low-temperature mode is shifted to the high-temperaturemode. That is, the setting temperature is changed to a value in thehigh-temperature region, and simultaneously, the setting potential ischanged to a value in the high-temperature region. Namely, this value isthe potential of the hypochlorite, which appears after completion of thedecomposition of the cyanic acid according to reaction (6), which hashigher inflection potential in the high-temperature reactions accordingto the equations (6) and (8). The potential of hypochlorite is usuallyapproximately 650 mV but may not necessarily be of this value dependingupon the waste liquor. This potential should therefore be preliminarilyconfirmed. When the reactions in the high-temperature region proceedwith the result that the measured potential reaches the settingpotential, then, dosing of the hypochlorite is stopped.

Single Step Method

In the present invention, with regard to the waste liquor, whichcontains cyanide, ferrocyanide and ammonia, and which has roomtemperature, the reactions (5), (7) and (9) as well as the reactions (6)and (8) can be successively carried out during the temperature-elevationstep from room temperature to high-temperature of from 80° C. to boilingpoint and during the temperature-holding step.

An embodiment of the single step method is illustrated in FIGS. 3 and 4.In FIG. 3, the abscissa is the treating time (minutes), the ordinate(left side) is the oxidation-reduction potential (mV), and the ordinate(right side) is the temperature (° C.). In FIG. 4, the abscissa is thetreating time (minutes), the ordinate (left side) is the concentration(mg/L) of CN, NH₃, and Fe, and the ordinate (right side) is pH. Thehypochlorite was continuously added so that the total amount is as highas 1.05 times the theoretical amount.

{circle over (1)} Decomposition of CN (total cyano matters). While thetemperature is elevated to approximately 60° C., the first-stepdecomposition corresponding to reaction equation (5) occurs in 10minutes. The CN concentration is subsequently kept constant during theperiod of 10˜25 minutes. Further, the CN concentration again decreasesalong with the decomposition of ferricyanide in the treatment after 25minutes.

{circle over (2)} NH₃ During the period of longer than 5 minutes and upto 45 minutes, this concentration continuously decreases along with thetemperature elevation from about 45° C. to the boiling point. Thiscorresponds to equation (9).

{circle over (3)} Oxidation Reaction of Ferrocyanide to Ferricyanide.This corresponds to equation (7) and does not show any change in the Feconcentration.

{circle over (4)} Decomposition Reaction of Ferricyanide. This reactioncorresponds to equation (8) and proceeds in the treating time of 25minutes or longer and results in a change in the Fe concentration.

{circle over (5)} Change in Oxidation-Reduction Potential (ORP). Theoxidation-reduction potential gradually increases and becomes a constantvalue at 50 minutes or later. In the present experiment, theoxidation-reduction potential of hypochlorite is preliminarily evaluatedto be 670 mV. Addition of hypochlorite is continued up to 50 minutes. Asan alternative method, the constant oxidation-reduction potential isconfirmed in 50˜60 minutes, and the addition of hypochlorite may beterminated at 60 minutes. In this method, the hypochlorite added in theperiod of 50˜60 minutes is excessive relative to equation (9).

{circle over (6)} Color Change of Waste Liquor. After a lapse of 10minutes, brown coloring starts. After a lapse of 20 minutes, red-brownprecipitation occurs. Subsequently, the color changes from red, to brownand to yellow. Through this change, the complete decoloration totransparency occurs in 50 minutes.

{circle over (7)} Change in pH. This corresponds also to the change inoxidation-reduction potential, and pH lowers from 10.5 and arrives atconstant pH 9.5.

{circle over (8)} Temperature. Temperature rises from room temperatureto approximately 95° C. in 40 minutes. Subsequently, the temperature isheld at a constant level.

Theoretical Amount of Sodium Hypochlorite

The theoretical amount of sodium hypochlorite, which is required fordecomposing such main components of the salt-bath nitriding waste-liquoras free cyano matter, cyanic acid, ammonia and ferrocyanate ions, is asshown in Table 2 following the equations (5)˜(9). In the presentinvention, the sodium hypochlorite can be continuously or intermittentlyadded within the treating time of, for example, 60˜120 minutes, so thatthe total addition amount is 1.01˜1.05 times as high as the theoreticalamount. TABLE 2 Using Amount of Material NaClO (Theoretical Amount) FreeCyano Matter 7.16 g/CN(1 g) CNO⁻ 2.66 g/CNO⁻(1 g) NH₃ 7.98 g/CN(1 g)Fe(CN)₆ ⁴⁻ 7.40 g/CN(1 g)

Regulated Value of Waste Water

The regulated value of total cyano matters according to the waste-waterstandard varies in the countries. In most of the occidental states andJapan, the regulated value is 1 ppm or less. In Asian states, theregulated value is 0.5 ppm or less in China, 0.2 ppm or less inThailand, and 0.04 ppm or less in Indonesia.

Under the circumstances described above, it is urgent to establish sucha treatment method to be implemented in Asian regions that at least 0.1ppm can be ensured.

Detoxification of Solid Discard

It is necessary in the salt-bath nitriding not only to treat the rinsingwaste-liquid but also to periodically remove sludge, i.e., the insolubleiron compound which increases in the salt bath along with the treatment.Consequently, solid discard including cyanide generates. This soliddiscard (hereinafter referred to as “the removed sludge”) is thespecifically regulated industrial discard and its treatment is usuallyconsigned outside. The removed sludge has, therefore, been an impedimentto the achievement of a toxic-matter free plant. Since the removedsludge is pumped out from the salt bath, it is mainly composed of thecomponents of the salt bath and is water-soluble.

The present inventors make it possible to attain almost zero dischargingof the specifically regulated industrial discard from a salt bathnitriding-plant by means of dissolving and dispersing the removed sludgein the rinsing waste-water and subjecting it to the detoxificationtreatment according to the present invention.

Decomposition of Cyano Complex of Valuable Metals

Such valuable metals as Au, Ag, Cu and Zn are plated even today by usingthe cyano plating-bath. Such cyano complexes as Ag, Cu, Zn and the likemay be contained in the plating liquor, together with the cyano complexof Au, i.e., the treating subject of the present invention. Since everyone of these metal cyano-complexes has a higher dissociation constantthan that of the iron cyano-complex, NaClO can decompose CN of theformer complex under an alkaline state and at high temperature. TABLE 3Chemical Form Complex Ion Dissociation Constant Na₂Zn(CN)₄ Zn(CN)₄ ²⁻1.3 × 10⁻¹⁷ Na₂Cd(CN)₄ Cd(CN)₄ ²⁻ 1.4 × 10⁻¹⁹ Na₃Ag(CN)₄ Ag (CN)₄ ³⁻ 2.1× 10⁻²¹ Na₂Ni(CN)₄ Ni (CN)₄ ³⁻ 1.0 × 10⁻²² Na₃Cu(CN)₄ Cu(CN)₄ ³⁻ 5.0 ×10⁻²⁸ Na₂Au(CN)₄ Au(CN)₄ ³⁻ 5.0 × 10⁻³⁹ K₄Fe(CN)₆ Fe(CN)₆ ⁴⁻ 1.3 × 10⁻³⁵K₄Fe(CN)₆ Fe(CN)₆ ³⁻ 1.3 × 10⁻⁴²

When sodium hypochlorite is added to the cyano plating waste-liquorcontaining valuable metals, reactions as shown in equations (10) through(21) proceed, at high temperature of 80° C. or more.

1) Ag Cyano ComplexNa₃Ag(CN)₄+4NaClO→AgCl+4NaCNO+3NaCl  (10)2NaCNO+3NaClO+H₂O→2NaHCO₃+N₂+3NaCl  (11)(10)+(11)Na₃Ag(CN)₄+10NaClO+2H₂O→AgCl+4NaHCO₃+2N₂+4NaOH+9NaCl  (12)

2) Monovalent Metal-Ion ComplexNa₃Me(CN)₄+4NaClO+NaOH→Me(OH)+4NaCNO+4NaCl  (13)2NaCNO+3NaClO+H₂O→2NaHCO₃+N₂+3NaCl  (14)(13)+(14)Na₃Me(CN)₄+10NaClO+NaOH+2H₂O→Me(OH)+4NaHCO₃+2N₂+10NaCl  (15)2Me(OH)→Me₂O+H₂O  (16)

3) Divalent Metal Ion ComplexNa₂Me(CN)₄+4NaClO+2NaOH→Me(OH)₂₊₄NaCNO+4NaCl  (17)2NaCNO+3NaClO+H₂O→2NaHCO₃+N₂₊₃NaCl  (18)(17)+(18)Na₂Me(CN)₄+10NaClO+2H₂O+2NaOH→Me(OH)₂+4NaHCO₃+2N₂+10NaCl  (19)Me(OH)₂→MeO+H₂O  (20)

4) Au Cyano ComplexNa₃Au(CN)₄+4NaClO→NaAuCl₂+4NaCNO+2NaCl  (21)4NaCNO+6NaClO+2H₂O→2N₂+4CO₂+4NaOH+6NaCl  (22)Na₃Au(CN)₄+10NaClO+2H₂O→NaAuCl₂+2N₂+4NaHCO₃+8NaCl  (23)

In the present invention, gold is neither precipitated nor deposited inthe form of hydroxide or oxide under the co-presence of an oxidizingagent, and is present in the solution in the form of gold chloric acid.Contrary to this, silver and most of the other metals are present in theform of silver chloride or metal oxide. Gold can therefore be separatedalone.

The reaction equations are the same as in the ordinary alkali chlorinemethod and consist of the first oxidation according to equations (10),(13), (17) and (21) in which cyano matter is converted to cyanic acid,and the second oxidation according to equations (11), (14), (18) and(23), in which the cyanic acid is converted to nitrogen gas andcarbonate. These reactions occur continuously in most cases under hightemperature. Equations (10)˜(12) indicate that the Ag cyano complexreacts as described above and precipitates and deposits as AgCl. Themonovalent metal and divalent metal are converted under high temperatureand copresence of oxidant to metal oxide as shown in equations (16) and(20), respectively. From equations (12), (15) and (19), it turns outthat 4 moles of CN and 10 moles of NaClO react. That is, 7.16 g of NaClOis necessary for 1 g of CN.

The present invention is hereinafter described with reference to theexamples.

Best Mode for Carrying Out Invention

Rinsing waste liquor discharged from a salt-bath nitriding line can betreated by means of the apparatus, the flow chart of which is shown inFIG. 5, utilizing the single-step method and the two-step methodaccording to the present invention.

In the drawing, 1 is a storage tank of the original waste liquor, 2 is areagent tank (hypochlorite solution), 3 is a reagent tank (sulfuric acidtank), 4 is a reaction tank, 5 is a tank of treating liquor combinedwith a neutralizing tank, 6 is a control panel. 7 is a heater forheating the treating liquor. The heater 7 used in the present exampledoesnot limit the heating apparatus, as long as it can be effectivelyused in the place of installation. For example, either vapor or gas maybe used. 8 is a stirrer. 9 is a duct which is connected with a scrubberfor collecting the generated ammonia gas. 10, 11 and 12 are sensors formeasuring the temperature, pH and ORP, respectively. 13, 14, 16˜18 areliquor-conveying pumps. The pump 18 is preferably a continuous flowpump. The reaction tank 4 is of closed structure.

EXAMPLE 1

The two-step method according to the present invention was carried outusing the apparatus, the flow chart of which is shown in FIG. 5, withregard to the rinsing waste-liquor, which was discharged from thesalt-bath nitriding line.

The analysis results of the rinsing waste-liquor are shown in Table 4.

The above-mentioned waste liquor in an amount of 1 m³ was admitted inthe reaction tank 4 of FIG. 5. The treatment was started at the settingtemperature of 79° C. and setting potential of 350 mV. At the start, thetemperature and oxygen-reduction potential (ORP) were 25° C. and −71 mV,respectively.

When the measured potential was lower than the setting potential, 10%sodium-hypochlorite aqueous solution was intermittently dosed from thereagent tank 2 into the reaction tank 4 by means of the continuous flowpump 18 having 10 L/minute of capacity. The temperature of the liquorreached the setting temperature of 79° C. after 25 minutes from thestart. The oxygen-reduction potential (ORP) reached the settingpotential after 30 minutes from the start.

When the measured potential reached the setting potential, the settingtemperature was changed to 95° C. and the setting potential was changedto 600 mV, in order to shift to the high-temperature mode. The 10%sodium-hypochlorite aqueous solution was intermittently dosed by meansof the continuous flow pump 18. The oxidation-reduction potential (ORP)reached 600 mV in 95 minutes in total from the start. In order toachieve the decomposition of sodium hypochlorite present in smallexcess, the heating was stopped and only the stirring was continued for25 minutes. Subsequently, the liquor was transferred by means of thepump 16 into the neutralizing tank 5, and 10% sulfuric acid was dosedfrom the reagent tank 3 into the neutralizing tank 5 to provide pH 8.5.The supernatant liquor was sampled for analysis. The results of theexperiment are described in Table 4.

The analytical results show that the total cyano matters are less than0.1 mg/L, and further ammoniacal nitrogen was not detected. The dosingamount of the sodium hypochlorite was 1.03 times as high as thetheoretical value calculated from the analytical value of the wasteliquor.

EXAMPLE 2

The two-step method according to the present invention was carried outusing the apparatus, the flow chart of which is shown in FIG. 5, withregard to the rinsing waste liquor, which was discharged from thesalt-bath nitriding line.

The analysis results of the rinsing waste liquor are shown in Table 4.

The above-mentioned waste liquor in an amount of 1 m³ was admitted inthe reaction tank 4 of FIG. 5. The treatment was started at the settingtemperature of 75° C. and setting potential of 400 mV. At the start, thetemperature and oxygen-reduction potential (ORP) were 24° C. and −65 mV,respectively.

When the measured potential was lower than the setting potential, the10% sodium-hypochlorite aqueous solution was intermittently dosed fromthe reagent tank 2 into the reaction tank 4 by means of the continuousflow pump 18 having 10 L/minute of capacity. The temperature of theliquor reached the setting temperature of 75° C., after t23 minutes fromthe start.

When the measured potential reached the setting potential, the settingtemperature was changed to 95° C. and the setting potential was changedto 600 mV, in order to shift to the high-temperature mode. Theoxidation-reduction potential (ORP) reached 600 mV in 100 minutes intotal from the start. In order to achieve the decomposition of sodiumhypochlorite present in small excess, the heating was stopped and onlythe stirring was continued for 20 minutes. Subsequently, the liquor wastransferred by means of the pump 16 into the neutralizing tank 5, and10% sulfuric acid was dosed from the reagent tank 3 into theneutralizing tank 5 to provide pH 8.5. The supernatant liquor wassampled for analysis. The results of the experiment are described inTable 4.

The analytical results show that the total cyano matters are less than0.1 mg/L, and further ammoniacal nitrogen was not detected (indicated asND in the table). The dosing amount of the sodium hypochlorite was 1.05times as high as the theoretical value calculated from the analyticalvalue of the waste liquor.

EXAMPLE 3

Removed sludge in an amount of 50 kg from the rinsing step of thenitriding salt-bath, was dissolved in the waste liquor from the rinsingprocess to provide waste liquor in an amount of 1 m³. The two-stepmethod according to the present invention was carried out.

This waste liquor in an amount of 1 m³ was admitted in the reaction tank4 of FIG. 5. The analysis results of the rinsing waste liquor are shownin Table 4.

The treatment was started at the setting temperature of 70° C. andsetting potential of 330 mV. At the start, the temperature andoxygen-reduction potential (ORP) were 25° C. and −52 mV, respectively.

When the measured potential was lower than the setting potential, the10% sodium-hypochlorite aqueous solution was intermittently dosed fromthe reagent tank 2 into the reaction tank 4 by means of the continuousflow pump 18 having 10 L/minute of capacity. The temperature of liquorreached setting temperature of 70° C., after 20 minutes from the start.

When the measured potential reached the setting potential, the settingtemperature was changed to 85° C. and the setting potential was changedto 650 mV, so as to shift to the high-temperature mode. Theoxidation-reduction potential (ORP) reached 650 mV in 93 minutes intotal from the start. In order to achieve decomposition of sodiumhypochlorite present in small excess, the heating was stopped and onlythe stirring was continued for 7 minutes. Subsequently, the liquor wastransferred by means of the pump 16 into the neutralizing tank 5, and10% sulfuric acid was dosed from the reagent tank 3 into theneutralizing tank 5 to provide pH 7.7 The supernatant liquor was sampledfor analysis. The results of the experiment are described in Table 4.

limits of detection, and further ammoniacal nitrogen was not detected.The dosing amount of the sodium hypochlorite was 1.05 times as high asthe theoretical value calculated from the analytical value of the wasteliquor. TABLE 4 Items Example 1 Example 2 Example 3 Before Treatment PH11.4 11.0 11.1 Free Cyano Matters (mg/L) 2200 500 250 Total Free Cyanomatters (mg/L) 3250 1200 880 Cyanic Acid (mg/L) 3200 3000 3320Ammoniacal Nitrogen (mg/L) 1380 290 300 Total Nitrogen (mg/L) 1860 500650 Treatment Low-Temperature Range Setting 79 75 70 ConditionsTemperature (° C.) High-Temperature Range Setting 95 95 85 Temperature(° C.) Low-Temperature Range Setting 350 400 330 Potential (mV)High-Temperature Range Setting 600 600 650 potential (mV) Dosage Amountof Sodium 1.03 1.05 1.05 Hypochlorite (Relative Amount of TheoreticalAmount) Treating Time (minutes) 120 120 120 After Treatment PH 8.4 8.57.7 Total Cyano Matters (mg/L) ND ND ND Ammoniacal Nitrogen (mg/L) ND NDND Total Nitrogen (mg/L) 200 90 0

REFERENCE EXAMPLE 1

The silver-cyanide plating waste-liquor having the composition shown inTable 5, discharged from the plating plant, was subjected to thedetoxification treatment according to the two-step method of the presentinvention. The plating waste liquor in an amount of 1 m³ was admitted inthe reaction tank 4 of FIG. 5.

The treatment was started at the setting temperature of 79° C. and thesetting potential of 250 mV. At the start, the temperature andoxygen-reduction potential (ORP) were 23° C. and −92 mV, respectively.

When the measured potential was lower than the setting potential, the10% sodium-hypochlorite aqueous solution was intermittently dosed fromthe reagent tank 2 into the reaction tank 4 by means of the continuousflow pump 18 having 10 L/minute of capacity. The temperature of theliquor reached the setting temperature of 79° C., after 26 minutes fromthe start. The oxygen-reduction potential (ORP) reached the setting 26minutes from the start. The oxygen reduction potential (ORP) reached thesetting potential after 30 minutes from the start.

When the measured potential reached the setting potential, the settingtemperature was changed to 95° C. and the setting potential was changedto 660 mV, so as to shift to the high-temperature mode. The 10% sodiumhypochlorite aqueous solution was continuously added by the continuousflow pump 18. The oxidation-reduction potential (ORP) reached 660 mV in95 minutes in total from the start. In order to achieve thedecomposition of sodium hypochlorite present in small excess, theheating was stopped and only the stirring was continued for 25 minutes.Subsequently, the 10% sulfuric acid was dosed for neutralizing into tank5 to provide pH 8.0. The supernatant liquor was sampled for analysis.The results of experiment were described in Table 5.

The analytical results shows that the total cyano matters are less than0.1 mg/L, and further ammoniacal nitrogen was not detected. The dosingamount of the sodium hypochlorite was 1.02 times as high as thetheoretical value calculated from the analytical value of the wasteliquor. TABLE 5 Items Example 4 Before pH 12.5 Treatment Total FreeCyano Matters (mg/L) 10600 Silver (mg/L) 4900 Total Nitrogen (mg/L) 550Treatment Low-Temperature Range Setting 79 Conditions Temperature (° C.)High-Temperature Range Setting 95 Temperature (° C.) Low-TemperatureRange Setting Potential (mV) 250 High-Temperature Range SettingPotential (mV) 660 Dosage Amount of Sodium Hypochlorite 1.02 (RelativeAmount to Theoretical Amount) Treating Time (minutes) 120 After pH 8.0Treatment Total Cyano Matters (mg/L) ND Ammoniacal Nitrogen (mg/L) 0.5Total Nitrogen (mg/L) ND

COMPARATIVE EXAMPLE 1

As a comparative example, the rinsing waste-liquor, which was dischargedfrom the salt-bath nitriding line, was treated by the conventionalmethod, i.e., the excess chlorine method (the method, in which excessoxidizing agent more than the theoretical method is dosed in a constantamount at the beginning, referred to in “Techniques and Rules forPreventing Environmental Pollution” (ditto)).

The waste liquor in 5 L, which belonged to the same lot as that used inExample 1, was admitted in a stainless-steel beaker in 10 L. While thewaste liquor was being stirred with a stirrer, the sodium hypochloriteaqueous solution (10% solution) in an amount as high as 1.05 times thatcalculated from the analytical value was charged into the waste liquor.Temperature-rise was then started. After 20 minutes, the temperaturereached the predetermined temperature 90° C. The temperature was thenmaintained for 100 minutes to perform the oxidizing decomposition (thereaction time being 120 minutes).

After the completion of oxidizing decomposition, the supernatant liquorwas sampled for analysis. The analysis results of the treating liquorare shown in Table 6.

The analytical results show that the total cyano matters of the liquor,which has been treated by the above method, was 26.0 mg/L more than thewaste-liquor regulating value of 1 mg/L. Although the ammoniacalnitrogen was not detected, since the ammonia odor was detected duringtreatment, it is clear that a part of the ammonia vaporized into theatmospheric air.

COMPARATIVE EXAMPLE 2

The waste liquor in 5 L, which belonged to the same lot as that used inExample 1, was admitted in a stainless-steel beaker in 10 L. While thewaste liquor was being stirred with a stirrer, sodium hypochloriteaqueous solution (10% solution) in an amount as high as 1.5 times thatcalculated from the analytical value was charged into the waste liquor.Temperature-rise was then started. After 20 minutes, the temperaturereached the predetermined temperature 90° C. The Temperature was thenmaintained for 100 minutes to perform the oxidizing decomposition.

After the completion of oxidizing decomposition, the supernatant liquorwas sampled for analysis. The analysis results of the treating liquorare shown in Table 6.

The analytical results show that the total cyano matters of the liquor,which has been treated by the above method, was 3.0 mg/L more than thewaste-liquor regulating value of 1 mg/L. Although the ammoniacalnitrogen was not detected, since the ammonia odor was detected duringtreatment, it is clear that a part of the ammonia vaporized into theatmospheric air.

COMPARATIVE EXAMPLE 3

The waste liquor in 5 L, which belonged to the same lot as that used inExample 1, was admitted in a stainless-steel beaker in 10 L. While thewaste liquor was amount as high as 2.0 times that calculated from theanalytical value was charged into the waste liquor. Temperature-rise wasthen started. After 20 minutes, the temperature reached thepredetermined 90° C. Temperature was then maintained for 100 minutes toperform the oxidizing decomposition.

After the completion of oxidizing decomposition, the supernatant liquorwas sampled for analysis. The analysis results of the treating liquorare shown in Table 6.

The analytical results show that the total cyano matters of the liquor,which has been treated by the above method, was 0.1 mg/L less than thewaste-liquor regulating value of 1 mg/L. Although the ammoniacalnitrogen was not detected, since the ammonia odor was detected, it isclear that a part of the ammonia vaporized into the atmospheric air.

However, the dosing amount of sodium hypochlorite is twice as high asthe calculated value, which is economically disadvantageous as comparedwith the present invention. When the waste liquor is exhausted astreated, there is a danger of influence on the ecological system due tothe sterilization effect of chlorine. TABLE 6 Comparative ComparativeComparative Items Example 1 Example 2 Example 3 Before PH 11.4 11.4 11.4Treatment Free Cyano Matters (mg/L) 2200 2200 2200 Total Cyano Matters(mg/L) 3250 3250 3250 Cyanic Acid (mg/L) 3200 3200 3200 AmmoniacalNitrogen (mg/L) 1380 1380 1380 Total Nitrogen(mg/L) 1860 1860 1860Treatment Temperature (° C.) 90 90 90 Conditions Treating Time (minutes)120 120 120 Dosage Amount of Sodium 1.05 1.50 2.0 Hypochlorite (RelativeAmount of Theoretical Amount) After PH 9.7 9.5 9.5 Treatment Total CyanoMatters (mg/L) 26.0 3.0 <0.1 Ammoniacal Nitrogen (mg/L) ND ND ND TotalNitrogen (mg/L) 450 280 250

EXAMPLE 4

In this example, the water-rinse liquor of steel parts treated by thesalt-bath nitriding (Tuftriding) (hereinafter referred to as “Tuftridingrinse water”) (pH10.4) was treated by the single-step method accordingto the present invention. This Tuftriding rinse water contained 700 mg/Lof free cyano matters, 1160 mg/L of total cyano matters, 990 mg/L ofcyanic acid, 470 mg/L of N—NH₃, and 520 mg/L of total N. Most of thecyano matters, which correspond to the difference between the total andfree cyano matters, are present as the ferrocyanic complex. This wasteliquor in an amount of 1 m³ was admitted into the reaction tank 4. Whilethe waste liquor was heated at 1.5° C./minutes of temperature-elevatingspeed, the 12% sodium hypochlorite aqueous solution was charged at 100kg/hour of the charging rate until arrival of ORP potential at 640 mV(approximately 1.2 hours). After approximately 45 minutes, thetemperature reached 95° C., the temperature was then held. Subsequently,like in Example 1, stirring was carried out for 25 minutes, and thesupernatant liquor was sampled as in Example 1. The treated liquor in anamount of 1.1 m³ was obtained. The results are shown in Table 7.

EXAMPLE 5

The resultant sludge of the salt-bath nitriding (approximately 5% offree CN and approximately 30% of cyanic acid) was dissolved in theTuftriding rinse water at the concentration of 10% of the sludge. Theresultant dissolving liquor was treated by the single-step method of thepresent invention. This liquor to be treated contained 880 mg/L of totalCN (most is free CN), 3320 mg/L of cyanic acid (CNO), 300 mg/L of N—NH₃.This waste liquor in an amount of 1 m³ was admitted into the reactiontank 4. While the waste liquor was being heated at 1.5° C./minutes oftemperature-elevating speed, 12% sodium hypochlorite aqueous solutionwas charged at 100 kg/hour of the charging rate until the ORP potentialreached 640 mV after approximately 1.5 hours. The water-conveying pump18 was then stopped. Subsequently, the temperature was held at 85° C.and stirring was carried out for 30 minutes. Subsequently, the procedurelike in Example 1 was carried out, to obtain the treated liquor. Thetreated liquor in amount of 1.12 m³ was obtained. The results are shownin Table 7. TABLE 7 Example 1 Example 5 Example 6 Example 7 pH ofTreated Liquor 11.4 10.1 10.4 11.1 Concentration of Free CN of TreatedLiquor 2,200 3,600 700 — (mg/L) Concentration of Total CN (mg/L) 3,2504,810 1,160 880 Concentration of CNO (mg/L) 3,200 6,900 990 3,320Concentration of N—NH₃ (mg/L) 1,380 3,100 470 300 Total N Concentration(mg/L) 1,860 3,650 520 — Treated Amount (m³) 1 1 1 1 TreatingTemperature RT→ RT→ RT→ RT→ 95° C. 95° C. 95° C. 95° C. ORP Control 640mV 640 mV 640 mV 640 mV Dosage Speed of Reagents (kg/hour) 300 500 100100 Addition Amount of NaClO (Relative Ratio to 1.03 1.03 1.05 1.05Theoretical Amount) Treating Time (except for separation process 120 120120 120 of solid and liquid) minutes minutes minutes minutesPost-treatment pH 8.4 8.4 7.8 7.7 Post-treatment Total CNConcentration(mg/L) ND ND ND ND Post-treatment N—NH₃ Concentration(mg/L) ND ND ND ND Post-treatment N Concentration (mg/L) 200 280 40 —Post-treatment Fe Concentration (mg/L) 0.2 0.5 ND — Post-treatment CrConcentration (mg/L) 0.02 0.05 — — Post-treatment Ni Concentration(mg/L) 0.01 0.01 — —

REFERENCE EXAMPLE 2, EXAMPLES 6,7

The copper-cyanide plating solution containing Roschelle salt wasdetoxified by the single-step method of the present invention. Theresults are shown as Reference Example 2. The prepared waste liquor inExample 6 was a mixed liquor of the copper cyanide plating waste-liquorand the Tuftride waste liquor of Example 1. In every example, the cyanomatters were successfully detoxified as shown in Table 8. Particularly,organic matters incorporated into the waste liquor do not exert anyinfluence upon the detoxification treatment.

Example 7 is application to cyano matters of low-concentration. That is,the ten-times diluted solution of the Tuftride waste liquor mentionedabove was subjected to the detoxification, as well. The decompositionresults of the iron cyano complex at low concentration weresatisfactory. The solid-liquid separation was based on the coagulatingprecipitation method in Reference Example 2, and Examples 6,7. TABLE 8Reference Example 2 Example 6 Example 7 pH of Treated Liquor 12.5 11.610.6 Concentration of Free CN of Treated Liquor — — — (mg/L)Concentration of Total CN (mg/L) 10,700 5,760 410 Concentration of CNO(mg/L) — — — Concentration of N—NH₃ (mg/L) 350 650 120 Total NConcentration (mg/L) 550 770 150 Treated Amount (m³) 1 1 1 TreatingTemperature RT→95° C. RT→95° C. RT→95° C. ORP Control 660 mV 630 mV 600mV Dosage Speed of Reagents (kg/hour) 500 300 50 Addition Amount ofNaClO (Relative Ratio to 1.02 1.05 1.04 Theoretical Amount) TreatingTime (except for separation process of 120 120 100 solid and liquid)minutes minutes minutes Post-treatment pH 8.0 8.2 7.8 Post-treatmentTotal CN Concentration (mg/L) ND ND ND Post-treatment N—NH₃Concentration (mg/L) ND ND ND Post-treatment N Concentration (mg/L) 180160 15

COMPARATIVE EXAMPLES 4, 5, 6

In Comparative Example 4, the theoretical amount of the sodiumhypochlorite aqueous solution was charged in total at the initial periodafter heating. In Comparative Example 5, the total amount of the sodiumhypochlorite aqueous solution was added in total before heating, and thetreating temperature was 90° C. In Comparative Example 6, the treatingtemperature was 90° C., and the sodium hypochlorite aqueous solution wascharged until non-detection of CN. The reaction time was 120 minutesafter the temperature rise to 90° C., in every example. However, sincethe reaction tank was of open structure, the ammonia odor was detectedduring the temperature rise. That is, the ammonia was not treated byreactions, but was stripped from the reaction tank. The workingenvironment was detrimentally influenced, and the ammonia gas vaporizedinto the atmospheric air.

In each of Comparative Examples 4 and 5, the total CN value did not meetthe waste-water regulation value, i.e., 1 mg/L. It was recognized by ORPthat the hypochlorite remains after the treatment. In ComparativeExample 6, successful treatment was not attained unless sodiumhypochlorite is changed in an amount as high as twice the theoreticalamount. TABLE 9 Comparative Comparative Comparative Example 4 Example 5Example 6 pH of Treated Liquor 11.4 11.4 11.4 Concentration of Free CNof Treated Liquor (mg/L) 2,200 2,200 2,200 Concentration Total CN (mg/L)3,250 3,250 3,250 Concentration of CNO (mg/L) 3,200 3,200 3,200Concentration of N—NH₃ (mg/L) 1,380 1,380 1,380 Total N Concentration(mg/L) 1,860 1,860 1,860 Treated Amount (m³) 1 1 1 Treating Temperature95° C. 95° C. 95° C. Addition Amount of NaClO (Relative 1.05 1.05 1.9Ratio to Theoretical Amount) Treating Time (except for separation 120minutes 120 minutes 120 minutes process of solid and liquid)Post-treatment Total CN Concentration (mg/L) 81.5 26.0 ND Post-treatmentN—NH₃ Concentration (mg/L) ND ND ND Post-treatment Total N Concentration(mg/L) 620 450 250

REFERENCE EXAMPLE 3

The cyano-based copper-plating liquor in 200 mL was admitted in thereaction tank, as it was. It was heated to approximately 95° C.,followed by addition of the 10% sodium hypochlorite solution at a speedof 40 kg/h. The ORP potential was measured. The ORP potential varied in500˜800 mV as shown in FIG. 6. After preliminarily measuring this ORPchange curve, the potential was set at 600 mV. The sodium hypochloritewas continuously added to the identical plating solution under theidentical conditions until reaching the setting potential. Stirring wasthen carried out for 30 minutes. After standing still for 3 hours, thesupernatant liquor was analyzed. As shown in Table 2, CN was completelyanalyzed and black precipitates were formed. Cu₂O was identified by XRDanalysis of the precipitates. The recovery ratio of Cu is 99.8% or more,and the Cu₂O ratio in the precipitates was 93.5%.

EXAMPLE 8

The rinsing waste-water, which was discharged from the salt-bathnitriding line, was subjected to the treatment process of the presentinvention by means of an experimental apparatus, which virtually followsthe process-flow chart shown in FIG. 5.

The analytical results of the rinsing waste-water, which was dischargedfrom the salt-bath nitriding line, are shown in Table 10.

This waste-water in 100 L was admitted into a reaction vessel 4 providedwith a stirrer 8 and was heated by a heater 7. Temperature was elevatedfrom the room temperature to the setting temperature of 95° C. in 40minutes. The ORP potential at the room temperature (25° C.) was 28 mV.Along with the temperature rise, the potential was lowered to −88 mV atthe setting temperature of 95° C. When the temperature reached thesetting temperature, the dosing of the sodium hypochlorite (theeffective chlorine being 12%) was started in such a manner that it wasdosed from the reservoir tank 2 by means of the continuous flow pump 22.After dosing start, the inflection points were detected as follows: thefirst inflection point—0˜100 mV after 5˜6 minutes; the second inflectionpoint—300˜420 mV after 10˜12 minutes; and the third inflectionpoint—530˜590 mV after 40˜45 minutes. After passing these inflectionpoints, the potential was elevated to 600 mV, so that the dosing ofsodium hypochlorite was stopped. At this instance, the sampling foranalysis was taken. The liquor temperature was maintained at 95° C. andthe stirring was continued. An additional sampling was made after 10minutes and 30 minutes so as to investigate the effects of aging. Theheating and stirring were then stopped.

Red-brown sludge, which appeared to be ferric oxide, was accumulated onthe bottom of a tank; this was withdrawn, and the residual liquor wastransferred to the neutralizing precipitation tank. The liquor in theneutralizing precipitation tank was transferred to a heat exchanger tolower the temperature to 60° C. or less. The sodium bisulfite aqueoussolution (10% solution) was dosed from a reservoir tank (not shown) intothe liquor mentioned above until the ORP potential was lowered to 250 mVor less. The residual chlorine was decomposed. Subsequently, the dilutesulfuric acid was dosed from the reservoir tank 3 to adjust pH to avalue of 7˜8. Subsequently, polymer flocculant was dosed from areservoir tank (not shown) and the stirring was stopped, therebypromoting the formation of flocs. The sludge on the bottom was withdrawnand the residual liquor was transferred to a separate reservoir tank oftreated water. The treated water was subjected to the analysis of totalcyano matters. After confirming that the total cyano matters was withinthe regulated value, the treated water passed through a not-shownfiltration tower and guided through an effluent tower (not shown). Thus,the treatment was completed.

The results of experiments are shown in Table 10.

At the instance that the ORP-potential rise terminates, the dosing ofsodium hypochlorite was stopped. Subsequently, aging was performed insuch a manner that the temperature was maintained and stirring wascontinued for a predetermined time. Investigation of the aging effectsrevealed its great contribution.

In the case of without aging, the total cyano matters amounted to 6.8ppm and hence exceeded the regulated value of 1 ppm. The total cyanomatters decreased to 0.9 ppm and less than the detecting limit (0.1 ppm)by the aging for 10 minutes and 30 minutes, respectively.

The result shows that the dosing amount of sodium hypochlorite is lessthan the theoretical value, probably because ammonia was stripped duringthe temperature-elevation from the room temperature to 95° C.

Slurry was discharged from the bottom of a reaction tank and dried in adrying furnace. The resultant sludge in 0.2 g was weighed and analyzedby a method, which basically follows a method of JIS K0102/38 foranalyzing the total cyano matters. However, the phosphoric acid addedduring the distillation was 50 mL in order to completely dissolve thesludge., i.e., 40 mL in excess as compared with the ordinary method. Theresult was that the total cyano matters are less than the detectionlimit.

Judging from this result, the sludge, which generates along with theinventive treatment, can be disposed as the ordinary industrial discard.TABLE 10 Items Example 8-1 Example 8-2 Example 8-1 Before pH 11.4 11.411.4 Treatment Free Cyano Matters (mg/L) 2200 2200 2200 Total CyanoMatters (mg/L) 3250 3250 3250 Cyanic Acid (mg/L) 3200 3200 3200Ammoniacal Nitrogen (mg/L) 1380 1380 1380 Treatment Setting Temperature(° C.) 95 95 95 Conditions Reached Potentail (mV) 600 600 600 DosageAmount of Sodium 0.78 0.78 0.78 Hypochlorite (Relative Amount of 0 10 30Theoretical Amount) Aging Time (minutes) After pH 10.2 10.2 10.2Treatment Total Cyano Matters (mg/L) 6.8 0.9 ND Ammoniacal Nitrogen(mg/L) ND ND ND

Industrial Applicability

In the heat-treatment industry and plating industry, it has beenrequired to develop a detoxification treatment of the cyano waste-liquorcontaining a stable cyano complex, which method does not incurexcessively large investment cost and is reliable. In the presentinvention, the objective was attained in as much the investment cost isnot high and the method is reliable in the detoxification.

1. A treating method of cyano waste-liquor, wherein the waste liquorcontaining a cyano complex is oxidation-decomposed by means of addinghypochlorite under alkaline condition, characterized in that said wasteliquor, which contains at least one cyano complex of iron, cobalt andgold, is maintained within a temperature range of from 80° C. to boilingpoint, and further the hypochlorite is continuously or intermittentlyadded until the waste liquor arrives at the oxidation-reductionpotential of the hypochlorite.
 2. A treating method of cyanowaste-liquor, according to claim 1, characterized in that thehypochlorite is continuously or intermittently added into the wasteliquor, which further contains at least one of ammonia and free cyanomatter, within a temperature range lower than 80° C., until the wasteliquor arrives at 400 mV of the oxidation-reduction potential, and,subsequently, the temperature is elevated up to a range of from 80° C.to boiling point, and said hypochlorite is added until theoxidation-reduction potential of the waste liquor arrives at theoxidation-reduction potential of hypochlorite.
 3. A treating method ofcyano-waste-liquor according to claim 3, characterized in that thetemperature of waste liquor is elevated from the room temperature to atemperature within 80° to boiling point followed by maintaining thetemperature, and, further, the oxidation reduction potential of thewaste liquor is measured from the room temperature.
 4. A treating methodof cyano-waste liquor according to any one of claims 1 through 3,subsequently to detection of the oxidation potential of the hypochloritewithin a temperature range of from 80° C. to the boiling point, aging iscarried out by means of holding the temperature of said waste liquorwithin said temperature range.
 5. A treating method of the cyanowaste-liquor according to claim 1, wherein said waste liquor is rinsingwaste-liquor of steel parts treated by salt-bath nitriding.
 6. Atreating method of the cyano waste-liquor according to claim 1, whereinsaid waste liquor is cyano plating waste-liquor.